The invention relates to the field of p-type MOSFETS, and in particular to improving hole mobility in strained silicon p-type MOSFETS.
Strained silicon grown on relaxed Si1-xGex virtual substrates has been used to fabricate both n- and p-type MOSFETs, which exhibit enhanced carrier mobility compared to bulk silicon. Biaxial tensile strain breaks the six-fold degeneracy of silicon's conduction band, resulting in reduced intervalley scattering in nMOS devices. Furthermore, for in-plane transport, electrons have only the low transverse effective mass (mt=0.19m0). For ε-Si grown on Si0.8Ge0.2, the conduction band splitting is large enough to completely suppress intervalley scattering, and no further improvement in electron mobility is gained by increasing the strain in the silicon layer.
Biaxial tensile strain also splits the light-hole/heavy-hole degeneracy in the valence band. Unlike the conduction band, strain also changes the shape of the light-hole valley, resulting in lower in-plane and out-of-plane effective masses. Since the rate of subband splitting in the valence band is known to be lower than for the conduction band, theory predicts that intervalley scattering for holes will not be suppressed until the strain reaches 1.6%, corresponding to growth on a Si0.6Ge0.4 buffer. Recent experimental work shows that hole mobility enhancement saturates for ε-Si on Si0.6Ge0.4, with no further improvement as the virtual substrate Ge content was increased to 50%.
Unlike ε-Si nMOS, mobility enhancements in ε-Si p-type MOSFETs demonstrate a functional dependence on vertical effective field. While an 80% electron mobility enhancement has been observed in ε-Si for vertical fields ranging from 0.1 to 1MV/cm, hole mobility enhancements tend to evolve as the effective field changes, as shown in FIG. 1. For low fields, the hole mobility enhancement typically increases as the vertical field increases. At an intermediate field value, the enhancement peaks and then degrades with further increases in field.